The invention relates to an arrangement for mechanical loading of a bonding wire.
Bonding wires of thick aluminum wire are used for purposes of electrical contact-making of the power components of power modules for gate and emitter terminals or for wiring within the module. The bonding technique is ultrasonic bonding, the wires which are circular in cross section having a diameter of typically 200 to 300 microns and arcs with 10 to 20 mm width and up to 5 mm height being implemented. In order to be able to transmit high currents via a contact, several wires are made parallel.
In special mounting techniques (integrated converter) bonding wires are also used as connecting elements between power modules and circuit boards which contain control electronics. When using this converter technique in traction drives or in drives for machine tools under shock and vibration stress high mechanical accelerations occur with resulting forces which can cause a considerable shift of the bond base points. This causes deformation of the wire and formation of considerable stresses in the wire which when the elasticity limit is exceeded lead to plastic deformation and subsequently to material fatigue. With a sufficiently large number of these mechanical loading cycles, which typically lie in the range of 1 to 10 million cycles, ultimately operating failure occurs in the form of material fracture. This fracture which is known as a xe2x80x9cheel crackxe2x80x9d occurs preferably at the wire site damaged previously generally by bonding, specifically at the kink point at the bond base which defines one end of the wire.
For a long time there were no effective indicators which could predict premature fatigue of the bond connection and thus probable operating failure. To safeguard the reliability of bond connections in integrated mounting technique therefore very time-consuming and thus also expensive loading tests, for example shock and vibration tests, were used. Due to time consumption these tests can only be used to a limited extent for studies on objects with a long service life.
Wire fatigue under vibration loading was studied in the past within the framework of loading tests in so-called vibration machines with adjustable vibration frequency and amplitude. These machines however can be used conventionally only for certain frequency ranges, limited to certain sample sizes, and are expensive.
In addition to mechanical loading of the wire, during operation moreover strong thermal loading of the wire occurs. The wire is on the one hand heated via electrical losses in the wire, on the other hand the chip surface acts as a heat source (power cycle load case). Associated with this the wire undergoes a change of length due to thermal expansion; this leads to arching of the wire. This deformation causes similar stress conditions in the wires, such as a displacement of the base points. Maximum bending stress occurs again at the kink point at the bond base. Addition deformation is superimposed on this loading and results from the thermal mismatch between the material, for example aluminum, of the bonding wire, and the material, for example silicon, of the chip surface, or the material, for example copper, of another contact surface, and causes mechanical stresses at the contact site of the bond base. The latter mechanism after a relatively large number of temperature cycles leads to formation of cracks at the contact site of the wire with the contact surface and ultimately to lifting of the wire away from the contact surface. The danger of wire lifting can be reduced by coating the bond base with a protective layer. In the latter case the deformation of the wire during thermal expansion contributes mainly to fatigue and ultimately leads to xe2x80x9cheel crackxe2x80x9d.
In order to simulate the wire deformation which results during thermal expansion, K. V. Ravi. E. M. Philofsky in xe2x80x9cReliability improvement of wire bonds subjected to the fatigue stressesxe2x80x9d, 10th Proc. Reliab. Physics, Las Vegas, p. 143-148, 1972, suggested a test set-up in which the wire arc is periodically raised in the middle of the arc using a needle and in this way the arc height is increased. In this process the wire stress can lead to premature damage of the wire by the needle, pulling the wire can cause raising, and wire deformation differs considerably from the actual deformation upon thermal expansion.
H. Tomimuro, H. Jyumonji: xe2x80x9cNovel Reliability Test Method for Ribbon Interconnections between MIC Substratesxe2x80x9d, Proc. 36th Electronic Components Conference, Seattle, p. 324-330, 1986 discloses an electromechanical tester for thermomechanical fatigue of bonded gold bands. In this test the bonded ends of the gold bands are shifted up to 100 microns to or from one another via a piezoelectric actuator. Thus the thermomechanical loading of the gold band which forms in MICs (=Microwave Integrated Circuits) is simulated by the different coefficients of thermal expansion of the mounting materials during a temperature change.
Specifically this known tester is an arrangement for producing a mechanical load on a gold band which has
a carrier body,
a first contact surface which is fixed relative to the carrier body for attachment by bonding one end of the gold band,
a piezoelectric actuator which is mounted on a carrier body and which expands and contracts relative to the carrier body and in the direction to the first contact surface with a frequency of less than 1/60 Hz, and
a second contact surface attached to the actuator for attachment by bonding the other end of the gold band which moves back and forth upon expansion and contraction of the actuator relative to the carrier body and in the direction to the first contact surface with a frequency of less than 1/60 Hz of this expansion and contraction.
The gold band used in this known tester has a thickness of 20 microns and a width of 350 microns and thus a cross sectional shape which deviates sharply from the circular cross section of a bonding wire.
The ageing of a bonding wire by alternating temperature loading was studied in the past by so-called power cycles in which the components are periodically turned on and off. Based on the high thermal time constants, periods of 1-5 seconds are conventional so that service life tests can last a few weeks (see for example F. Auerbach, A. Lenninger: xe2x80x9cPower-Cycling-Stability of IGBT modulesxe2x80x9d, IEEE Industry Applications Society, New Orleans, p. 1248-1252, 1997).
The degree of fatigue of the bond connection is checked by various tests: the shear tensile strength in a pull test (destructive, non-destructive), the shear strength in a shearing test and the kinking behavior in an air jet test. Detailed information about crack formation and possible acceleration factors for ageing can be obtained via REM (scanning emission microscopy), ultrasonic tests and chemical analyses (for example, Auger spectroscopy).
It has already been suggested that wire fatigue during a service life test be described via reliability indicators. The electrical resistance, the nonlinearity of the electrical resistance and the resistance noise are examples of these indicators.
The object of the invention is to make available an arrangement for mechanical loading of a bonding wire which makes it possible to simulate the mechanical loading case of displacement of the base points of the bond connection under vibration or shock stress.
This object is achieved by the features of claim 1.
According to this approach an arrangement for mechanical loading of a bonding wire is made available which has:
a carrier body,
a first contact surface which is fixed relative to the carrier body for attachment by bonding one end of the bonding wire,
a piezoelectric actuator which is mounted on the carrier body and which can expand and contract relative to the carrier body and in the direction to the first contact surface with a frequency of at least 0.1 Hz,
a second contact surface attached to the actuator for attachment by bonding the other end of the bonding wire which moves back and forth during expansion and contraction of the actuator relative to the carrier body and in the direction to the first contact surface with the frequency of this expansion and contraction.
With the arrangement as claimed in the invention, advantageously in a very early stage of development accelerated service life tests for the bond connection can be carried out and thus critical parameters for reliability such as for example the wire material, arc geometry, wire guidance during bonding can be studied and evaluated early.
The arrangement as claimed in the invention advantageously allows the quality of the aforementioned reliability indicators which comprise for example the electrical resistance, the nonlinearity of the electrical resistance and the resistance noise for the loading case of mechanical deformation of the wire since measurement of the electrical resistance of the bond connection in this structure is possible.
Other advantages of the arrangement as claimed in the invention are:
It allows exact simulation of the displacements of the bond base points which occur during vibration and shock loading.
As a result of the high stress frequency, service life tests on mechanical loading and fatigue can be clearly accelerated.
The fatigue that accompanies deformation of the wire during thermal expansion can be simulated and due to the loading frequency which is higher compared to the performance cycle, accelerated stress tests can be carried out for this loading case.
The invention advantageously makes available an arrangement for producing a mechanical load on one bonding strip which has:
a carrier body,
a first contact surface which is fixed relative to the carrier body for attachment by bonding one end of the bonding strip,
a piezoelectric actuator means which is mounted on the carrier body and which can expand and contract independently in at least two different directions relative to the carrier body, and
a second contact surface attached to the actuator means for attachment by bonding the other end of the bonding strip which moves back and forth in that direction in which the actuator means expands and contracts relative to the carrier body.
The concept of bonding strip here means a strip-shaped material of any cross sectional shape and comprises both bonding wires and also bonding bands such as for example the aforementioned known gold band.
By means of this arrangement the ends or bond base points of one bonding strip which are attached to the first and second contact surface can be advantageously deflected in a controlled manner in at least two, preferably and advantageously in all three spatial directions and thus complex deflection processes can be simulated. Thus, for example, a change of the base point height which causes tilting of the arc of the bonding strip can be simulated. This loading case arises in the temperature cycle due to the different thermal coefficients of expansion of the mounting materials.
In one preferred and advantageous embodiment of this arrangement the actuator means has a sequence of two or more interconnected actuators, and
each actuator can expand and contract in each of one direction assigned only to it,
the first actuator of the sequence is mounted on the carrier body and expands and contracts in the direction assigned to it relative to the carrier body,
each actuator connected to the next actuator, when it expands and contracts in the direction assigned to it, moving this next actuator back and forth in this direction relative to the carrier body, and
the second contact surface being mounted on the last actuator of the sequence, and moving back and forth in this direction relative to the carrier body when the last actuator expands and contracts in the direction assigned to it.
Preferably and advantageously in this arrangement one direction points toward the fixed first contact surface.
It is especially advantageous if the actuator means can expand and contract at least in one direction, preferably at least in the direction pointing to the fixed first contact surface with a frequency of at least 0.1 Hz so that here too the second contact surface moves back and forth in the expansion and contraction of the actuator means relative to the carrier body with the frequency of this expansion and contraction.
Another advantageous arrangement as claimed in the invention for producing a mechanical load on the bonding strip has:
a carrier body,
a first contact surface which is fixed relative to the carrier body for attachment by bonding one end of the bonding strip,
a piezoelectric actuator means which is mounted on the carrier body and which can expand and contract at least in one direction relative to the carrier body, and
a second contact surface mounted on the actuator means for attachment by bonding the other end of the bonding strip which moves back and forth in one direction relative to the carrier body when the actuator means expands and contracts,
the second contact surface is held on a support body which is attached to the actuator means and which moves back and forth during expansion and contraction or the actuator means relative to the carrier body [sic]. In this arrangement moreover the features, especially all features of arrangements given above for producing a mechanical load on the bonding wire or strip can be implemented.
The support body preferably has one leg attached to the actuator means and one unsupported other leg on which the second contact surface is held.
The advantages of the arrangement as claimed in the invention include especially:
Simplified test execution compared to vibration machines due to the compact structure.
The displacement paths are adjustable to 1 micron in the piezoelectric actuators used. The maximum displacement path in these models is 50 microns.
By using piezoelectric actuators for lateral displacement of the bond base points, no hysteresis occurs in positioning, in contrast to electromechanical positioning elements (stepping motors). The vibration amplitude (wire deflection) is therefore unchanged even after millions of cycles.
The maximum vibration frequency is a few kHz. It is determined by the bandwidth of the piezoelectric actuators used and the resonant frequency of the arrangement, but with a suitable selection can be expanded to even higher frequencies. Stress tests can therefore be clearly accelerated.
Any time characteristic of deflection can be set (rectangular, ramp, sine functions) and thus complex loading cases can be studied.
The bond surfaces or contact surfaces are freely selectable (circuit board, DCB, bond pad on chip, etc.).
The geometry of the bond arc (width, height) and strip or wire material and strip or wire diameter can be freely selected.
A combination of the mechanical loading case with the loading of one performance cycle is possible. The bond arc can be stressed mechanically in the indicated arrangement and at the same time currents can be impressed or wire parameters such as the electrical resistance can be measured.
Any standard mounting techniques can be used as the second contact surface, for example copper-laminated circuit boards, DCB ceramics, chips with bond pads, etc. The lateral displacement of the bond base point attached to this contact surface can be adjusted via a high voltage which is applied to the piezoelectric actuator or the actuator means. In one test set-up at a maximum voltage of 1000 V displacement of 50 microns was achieved. The first contact surface is a surface, for example, a circuit board, which is located roughly 1 mm away from the movable surface and which is connected permanently to the carrier body for example in the form of a base plate. If the actuator or the actuator means is de-energized, it remains mechanically fixed on the carrier body. The bond base points cannot move therefore during and after production of the bond connection so that premature damage to the strip or wire can be excluded. The arrangement can be easily integrated into commercial bonders, for example, ultrasonic bonders, due to the compact structure.
Preferred applications of the invention are the power modules, integrated converters and chips on board.